Field of Endeavor
The present invention relates to an imaging spectrometer, and more to an arrangement of optical elements to provide full spectral imaging over a wide waveband, in this case covering the visible through infrared in a compact instrument.
State of Technology
The challenge in imaging spectrometers is to make them small enough to fit in the small unmanned aerial vehicles which are now available. For remote sensing purposes, these aerial vehicles present a savings in operating costs. Small Imaging spectrometers are also advantageous for man-portable instruments and for fitting into existing industrial production lines. Currently, reflective imaging spectrometers with convex gratings are typically used in the visible-infrared region. Making the convex gratings with low straylight for these visible-infrared spectrometers is difficult and expensive. Reflective imaging spectrometers are also too large for the smaller unmanned aerial vehicles.
U.S. Pat. No. 5,880,834 issued to Michael P. Chrisp Mar. 19, 1999 provides the following state of technology information: “There are three problems in designing an imaging spectrometer where light in a slice of an image field passing through an entrance slit is to be diffracted by a grating parallel to the slit and imaged onto a focal plane for display or recording with good spatial resolution parallel to the slit and good spectral resolution perpendicular to the slit: 1. Eliminating astigmatism over the spectrum on the image plane. 2. Removing field curvature from the spectrum focused onto the image plane. 3. Obtaining good spatial resolution of the entrance slit which involves eliminating astigmatism at different field angles from points on the entrance slit.” The disclosure of U.S. Pat. No. 5,880,834 is incorporated herein in its entirety for all purposes.
There are two methods for the making the convex gratings for the above imaging spectrometer, electron beam lithography and single point diamond turning. The gratings are difficult to make because for good diffraction efficiency the blaze angle needs to be constant to the local surface tangent, U.S. Pat. No. 6,480,333 discloses an electron beam fabrication method developed at the California Institute of technology. These gratings are extremely good, but having only one facility in the US has limited their application. Nowadays, most imaging spectrometers with convex gratings have them fabricated by single point diamond turning. Unfortunately these gratings tend to have high scattered light in the visible. Diamond turned mirror surfaces, when used in the visible are typically post polished to reduce the surface roughness, but with a grating surface this is impossible without destroying the grooves, so the intrinsic surface roughness is that generated by the diamond turning process.
A number of catadioptric designs have been developed for the infrared. However, these designs are based on two properties of infrared optical materials, high refractive index and low spectral dispersion, so they are not suitable of the visible-infrared spectral region.
U.S. Pat. No. 7,414,719 for an imaging spectrometer wide field catadioptric design issued to Michael P. Chrisp Aug. 19, 2008 provides the following state of technology information: “The present invention provides a compact imaging spectrometer with an immersive or reflective diffraction grating that compensates optical distortions. The imaging spectrometer comprises an entrance slit for transmitting light, a system with a catadioptric lens and a dioptric lens for receiving the light and directing the light, an immersion grating, and a detector array. The entrance slit, the system for receiving the light, the immersion grating, and the detector array are positioned wherein the entrance slit transmits light to the system for receiving the light and the system for receiving the light directs the light to the immersion grating and the immersion grating receives the light and directs the light through the system for receiving the light to the detector array. The compact imaging spectrometer uses smaller cryogenic coolers facilitating its using in portable (man carried) gas detection systems and in small unmanned aerial vehicles for remote gas detection.” The disclosure of U.S. Pat. No. 7,414,719 is incorporated herein in its entirety for all purposes.